6 APPARATUS AND METHODS, 



object dimly into focus, but to get more accurate focussing 

 (especially when using a high-power objective) we use the 

 fine adjustment, by turning a screw at the top of the 

 stand, behind the body-tube. 



The object to be examined is placed on the stage, which 

 has two clips for fixing a slide in a definite position, but 

 these need not be used except for high powers, or while 

 sketching. There is usually a black plate (diaphragm), 

 with holes of different sizes, under the stage ; this can be 

 rotated so as to bring the desired size of hole under the 

 central opening of the stage. More expensive microscopes 



Fig. 8. Double Nose-piece. 



have an iris diaphragm. A small hole is used with high 

 power and a large one with low power. 



For ordinary work two objectives are required, one for 

 low power (magnifying 60 to 80 diameters), and the other 

 for high power (300 to 400 diameters). The most useful 

 are 1 inch or | inch low-power objective and i or J inch 

 high-power objective. Two eye-pieces should also be used; 

 the one with shorter body and narrower eye-glass is the 

 more powerful. 



In most modern instruments the magnifying power can 

 be increased by having the body- tube constructed like a 

 telescope; the upper part (draw-tube), carrying the eye- 

 piece, can be drawn out. To avoid the inconvenience of 

 having to screw and unscrew a lens every time a change of 

 magnifying power is required, it is worth while to get a 

 nose-piece (Fig. 8), which is screwed to the lower end of 



